The present invention relates to the generation of the salt of a volatile base from a salt solution via treatment in an electrodialytic water splitter. The present invention has particular use in the generation and/or recovery of ammonium salts like ammonium sulfate at improved purity and concentration.
The use of water splitting to produce acid and base from salts is well known. Purification of acids and bases from solutions containing the acid or base is also known. However, processes which can be used to generate or isolate a desired salt at high concentration and purity have not been previously disclosed.
For example, in the production of zeolite catalyst supports, a solution of ammonium sulfate is used to ion exchange the zeolite from the sodium to the ammonium form. The zeolite process requires large quantities of concentrated, pure ammonium sulfate, and creates a large quantity of sodium sulfate/ammonium sulfate waste solution. A convenient sulfate salt, such as Na.sub.2 SO.sub.4 (or the mixed sulfate salt waste generated by the zeolite process) could be processed in a conventional electrodialytic water splitter to produce sulfuric acid and sodium hydroxide. The sulfuric acid could then be reacted with ammonia to produce ammonium sulfate. However, the electrodialytic production of acids, especially at high concentration, is particularly difficult, gives low current efficiency, and requires the use of high resistance membranes, which greatly increases the energy consumption of the electrodialytic unit.
Further the direct disposal of many industrial waste salts is environmentally objectionable. For example, in the zeolite process, large quantities of sodium/ammonium sulfate waste are generated, and are becoming increasingly more difficult to discard directly. Ammonium salts are particularly objectionable because they are nutrients which promote the growth of algea. Presently, the ammonium sulfate is recovered by adding caustic to the sodium/ammonium sulfate waste salt stream to free the ammonia, which is collected in an absorber. Sulfuric acid is added to the absorber, regenerating the ammonium sulfate. The remaining sodium sulfate generated is thrown away. Thus, even after recycling the volatile base a substantial quantity of salt is still discarded. Further, the caustic and sulfuric acid used are expensive, and add significantly to production costs.
Accordingly, an object of the present invention is to provide an improved electrodialytic process which can efficiently generate the salt of a volatile base, like ammonium sulfate, at high concentration and purity, from a related salt. A further object of the present invention is to provide a process which is capable of separating and concentrating the salt of a volatile base from a solution containing other related salts.
Three compartment electrodialytic water splitters are known in the art. They are disclosed to be comprised of alternating bipolar, anion and cation permselective membranes, thereby forming alternating acid, salt and base compartments. U.S. Pat. No. 2,829,095 discloses three compartment electrodialytic water splitters generally. U.S. Pat. No. 4,740,281 discloses the recovery of acids from materials comprising acid and salt using an electrodialytic three compartment water splitter to regenerate the acid from the salt.
Two compartment water splitters are also known in the art. U.S. application Ser. No. 278,062 discloses a two compartment electrodialytic unit for separating acid from a solution containing acid and salt.
Modified two compartment water splitters having an intermediate compartment which decreases unwanted migration of ions other than H.sup.+ and OH.sup.- are also known. Such water splitters are generally disclosed in U.S. application Ser. No. 626,643 (filed Dec. 12, 1990).
Chlanda, Water Splitting Efficiency of Bipolar Membranes, New Membrane Materials and Process for Separation, K. K. Sirkar and D. R. Lloyd, Eds, AICHE Symposia vol. 261, pgs. 64-71 (1988), discloses metering a base into the acid compartment of a three compartment water splitter as a means to measure the current efficiency of the cell.
As previously stated, use of any of the electrodialytic units to produce a concentrated salt require unsatisfactorily low current efficiency, and high power consumption. The concentration and purity of the salt generated by conventional methods is also unacceptably low. Accordingly, it is the primary objective of the present invention to provide an economical and efficient electrodialytic process capable of producing a salt, and particularly, the salt of a volatile base at high concentration and purity.